1. Field of the Invention
The present invention is directed to the field of radiation detection and more specifically to the area of achieving improved resolution in detector arrays.
2. Description of the Prior Art
A conventional range finder system is shown in FIG. 1, wherein a source of collimated electromagnetic radiation 22, such as a laser, emits a beam that is projected and scanned over a field of view. The laser 22 in combination with the lens system 20, provides a collimated beam of a predetermined cross-section that is partially reflected by a polarized beam splitter 18 along the system axis A.sub.T. The reflected polarized beam is rotated by a quarter wave plate 16 and directed towards a multi-faceted scanning mirror 14. Since the angle of incidence is continually changing due to the constant rotation of the scanning mirror 14, the resulting beam reflecting therefrom and the transmission axis A'.sub.T are repeatedly scanned across a defined linear path.
An elevation scanner 12 is located in the path of the linearly scanned beam from the scanning mirror 14 and provides an orthogonal scan vector to the beam as it is transmitted via an objective lens system 10. The resulting scan pattern is a raster type scan of a plurality of parallel line scans progressively swept over a field of view in an orthogonal direction controlled by the elevation scanner 12.
In the event a radiation reflective object (target) is located within the field of view, a portion of the transmitted beam incident thereon will be retroreflected towards the objective lens system 10. The received radiation (target return) is projected into the range finder system off-axis from the scanned transmission axis A'.sub.T. The objective lens system 10 projects the received radiation through the elevation scanner 12 towards the scanning mirror 14. Since the scanning mirror 14 is being rotated at a relatively high rate, the received radiation will have a different incident angle with respect to the mirror 14 than when it was transmitted. The rate of rotation for the mirror 14 is constant. Therefore, the change of incidence angle is related to the delay between transmitted and reflected radiation being incident thereon and a direct function of the distance between the mirror 14 and the target.
The received radiation polarization is rotated by the quarterwave plate 16 so that it is polarized parallel to the plane of polarization to the polarized beam splitter 18. The received radiation passes through the polarized beam splitter 18 and is focused onto a detector array 30 by a lens 24. The detector array is disposed with respect to the lens 24 so as to correspond to the scanned focal point of the system that is line scanned with the rotation of the scanning mirror 14.
The individual elements of the detector array 30 are illustrated in FIG. 2, corresponding to the line scan and the predicted amount of angular offset for target returns throughout a range of target distance. Each element represents a separate range bin and is connected to a signal processor 40 that provides appropriate range information according to the particular elements or group of elements that are illuminated. The range is determined by the angular relationship of .theta.=.theta.(2R/C). In that relationship, .theta. is the angle of offset of a returned radiation from the optical axis reference A.sub.R ; .theta. is the rate of scan of the optical axis A.sub.T contributed by the scanning mirror 14 rotating at the rate of .theta./2; C is the speed of light; and R is the range of the reflecting object from the scanning mirror 14.
The individual range elements in the detector array 30 are characterized as having equal height and width measurements with the relationship of 1.6.times..lambda./D resolution capability of the optics. In that relationship, .lambda. is the wavelength of the transmitted electromagnetic radiation and D is the aperture dimension of the objective lens 10.
The response characteristics of the conventional detector array are shown in FIG. 3 as a plot of the receiver relative signal power, for three adjacent elements "n-1", "n", and "n+1" representing separate range bin locations, versus the range of the target for an optimized return signal that would approximately cover the area of one range bin. From such a plot, it can be seen that the off-center performance for receiver relative signal power at the "n" detector drops to less than one-half (from 0.303 to 0.142) when the range is mid-way between range bins (detector elements). In addition, the receiver relative signal power at the "n" detector is decreased to a insignificant level (0.012) when the return signal is a full range bin interval away (e.g., at "n+1" or "n-1"). Accordingly, in that event, the signal-to-noise ratio at the "n" detector element would be 14 dB down with respect to the "n+1" or "n-1" . Such a large drop in signal-to-noise ratios is undesirable since the interpolation accuracy for off-centered detection is greatly degraded.